[R] How to do leave-n-out cross validation in R?

[Lily]

Seems crossval from library(bootstrap) can only be
used for leave-one-out and k-fold cross validation?
Here is a dumb question, suppose n=80, how to do
exactly leave-50-out cross validation? K-fold cross
validation is not eligible for this case since
n/ngroup is not an integer. Thanks!